There has been substantial interest in exploiting the properties of compound semiconductors consisting of particles with dimensions in the order of 2-50 nm, often referred to as quantum dots (QDs), nanoparticles, and/or nanocrystals. These materials are of commercial interest due to their size-tuneable electronic properties which can be exploited in many commercial applications such as optical and electronic devices and other applications that ranging from biological labelling, photovoltaics, catalysis, biological imaging, light-emitting diodes (LEDs), general space lighting and electroluminescent displays amongst many new and emerging applications.
The most studied of semiconductor materials have been the chalcogenides, II-VI materials namely ZnS, ZnSe, CdS, CdSe, CdTe; most notably CdSe due to its size-dependent tuneability over the visible region of the spectrum. Reproducible methods for the large scale production of these materials have been developed from “bottom up” techniques, whereby particles are prepared atom-by-atom, i.e. from molecules to clusters to particles, using “wet” chemical procedures.
Two fundamental factors, both related to the size of the individual semiconductor nanoparticle, are responsible for their unique properties. The first is the large surface area to volume ratio. Smaller particles have a larger ratio of surface atoms to non-surface atoms than larger particles. Thus, surface properties play a more important role in the overall properties of smaller particles. A second factor is that quantum confinement effects in small particles influence the electronic properties of the material in a size-dependent manner. The band gap gradually becomes larger as the size of the particle decreases. This effect is a consequence of the confinement of an “electron in a box”, giving rise to discrete energy levels similar to those observed in atoms and molecules, rather than a continuous band as observed in the corresponding bulk semiconductor material. Thus, for a semiconductor nanoparticle, the “electron and hole” produced by the absorption of electromagnetic radiation are closer together than they would be in the corresponding macrocrystalline material. As a consequence, Coulombic interaction cannot be neglected. This leads to a narrow bandwidth emission peak that is dependent upon the particle size and composition of the nanoparticle material. Thus, quantum dots have higher kinetic energy than the corresponding macrocrystalline material and, consequently, the first excitonic transition (band gap) increases in energy with decreasing particle diameter.
As used herein, the term “core semiconductor nanoparticle” and “core nanoparticle” refers to a nanoparticle of a single semiconductor material possibly coated by an organic passivating layer. Such core semiconductor nanoparticles tend to have relatively low quantum efficiencies due to electron-hole recombination occurring at defects and dangling bonds situated on the nanoparticle surface, resulting in non-radiative electron-hole recombinations. One method to eliminate defects and dangling bonds on the inorganic surface of the quantum dot is to grow a “shell” inorganic material on the core nanoparticle, producing a “core/shell” nanoparticle. The shell inorganic material preferably has a wider band gap than that of the core material and a lattice that matches closely with that of the core material. Core/shell particles separate charge carriers confined in the core from surface states that would otherwise act as non-radiative recombination centres. One example is a ZnS shell grown on the surface of a CdSe core. Another approach is to prepare a core/multishell structure where the “electron-hole” pair is confined to a single shell layer consisting of a few monolayers of a specific material, forming a quantum dot-quantum well structure. Here, the core is of a wide band gap material, followed by a thin shell of narrower band gap material, and capped with a further wide band gap layer, such as CdS/HgS/CdS grown using substitution of Hg for Cd on the surface of the core nanocrystal to deposit just a few monolayers of HgS, which is then over grown by a monolayer of CdS. The resulting structures exhibit clear confinement of photo-excited carriers in the HgS layer. To add further stability to quantum dots and help to confine the electron-hole pair one of the most common approaches is to grow a compositionally graded alloy layer on the core, which helps to alleviate strain resulting from the lattice mismatch between the core and shell materials that could otherwise lead to defects. Moreover, for a CdSe core, in order to improve structural stability and quantum yield, rather than growing a shell of ZnS directly on the core, a graded alloy layer of Cd1-xZnxSe1-ySy can be used. This has been found to greatly enhance the photoluminescence emission of the quantum dots.
The emission and absorption properties of nanoparticles can be manipulated by doping nanoparticles with atomic impurities. Procedures for doping wide band gap materials, such as zinc selenide and zinc sulfide, with manganese and copper (ZnSe:Mn or ZnS:Cu) have been developed. Doping with different luminescence activators in a semiconducting nanocrystal can be used to tune the photoluminescence and electroluminescence at energies even lower than the band gap of the bulk material, whereas the quantum size effect can be used to tune the excitation energy with the size of the quantum dots without having a significant change in the energy of the activator related emission.
A particularly attractive potential field of application for quantum dots is in the development of next generation LEDs. LEDs are becoming increasingly important in modern day life and it is envisaged that they have the potential to become one of the major applications for quantum dots, in for example, automobile lighting, traffic signals, general lighting, liquid crystal display (LCD) backlight units (BLUs) and display screens.
Currently, LED devices are made from inorganic solid-state compound semiconductors, such as AlGaAs (red), AlGaInP (orange-yellow-green), and AlGaInN (green-blue). However, solid-state LEDs that emit white light cannot be produced using available solid-state compound semiconductors. Moreover, it is difficult to produce “pure” colours by combining solid-state LEDs of different frequencies. At present, the main method of colour mixing to produce a required colour, including white, is to use a combination of phosphorescent materials placed on top of a solid-state LED. In such a configuration the light from the LED (the “primary light”) is absorbed by the phosphorescent material and then re-emitted at a second frequency (the “secondary light”). In other words, the phosphorescent materials down-convert the primary light to secondary light. White LEDs produced by phosphor down-conversion cost less and are simpler to fabricate than combinations of solid-state red-green-blue LEDs.
Current phosphorescent materials used in down-converting applications absorb predominantly UV or blue light and convert it to longer wavelengths. Most current phosphors are based on trivalent rare-earth-doped oxides or halophosphates. White emission can be obtained by blending phosphors that emit in the blue, green and red regions and stimulating such a blend with a blue or UV emitting solid-state device. A blue LED in combination with a green phosphor, such as SrGa2S4:Eu2+, and a red phosphor, such as SrSiEu2+, or a UV LED in combination with a yellow phosphor, such as Sr2P2O7:Eu2+; Mn2+, and a blue-green phosphor can produce white light. White LEDs can also be made by combining a blue LED with a yellow phosphor, but colour control and colour rendering is poor due to lack of tunability of the LEDs and the phosphor. Unfortunately, conventional LED phosphor technology used as down-converting materials produce light having poor colour rendering (i.e. a colour rendering index (CRI) <75).
The use of nanoparticle quantum dots potentially has some significant advantages over the use of the more conventional phosphors. One advantage is the ability to tune the emission wavelength of the nanoparticle phosphor by manipulating the particle size. Additionally, nanoparticle quantum dots exhibit strong absorption properties, narrow emission bandwidth, and low scattering if the quantum dots are monodispersed. Rudimentary quantum dot-based light-emitting devices have been made by embedding colloidally produced quantum dots in an optically transparent (or sufficiently transparent) LED encapsulation medium, such as a silicone or an acrylate, which is then placed on top of a solid-state LED. Such encapsulation media are often permeable to oxygen, which degrades the performance of the quantum dots.
Multi-layer gas-barrier coatings can be formed via atomic layer deposition (ALD). For example alternating Al2O3 (deposited by ALD) and SiO2 (deposited by rapid, Al-catalysed ALD) layers on Kapton (poly(4,4′-oxydipenylene-pyromellitimide)) has been reported. [A. A. Dameron, S. D. Davidson, B. B. Burton, P. F. Carcia, R. S. McLean and S. M. George, J. Phys. Chem. C, 2008, 122, 4573] However, the reported water vapour transmission rate (WVTR) was not improved beyond that of a single-layer ALD Al2O3 gas-barrier. The same group described the MLD of poly(aluminium ethylene glycol) films on Al2O3 surfaces (deposited by ALD), using trimethyl aluminium (TMA) and ethylene glycol (EG) precursors. [A. A. Dameron. D. Seghete, B. B. Burton, S. D. Davidson, A. S. Cavanagh, J. A. Bertrand and S. M. George, Chem. Mater., 2008, 20, 3315] The hybrid inorganic-organic polymer (alucone) film was found to be unstable in ambient conditions, with a decrease in film thickness commencing around 150 hours post-deposition.
For the commercial application of quantum dots in next-generation light-emitting devices, the quantum dots should be incorporated into the LED encapsulating material while remaining as fully monodispersed as possible and without significant loss of quantum efficiency. The methods developed to date are problematic, not least because of the nature of current LED encapsulants. Quantum dots can agglomerate when formulated into current LED encapsulants, thereby reducing the optical performance of the quantum dots. Moreover, even after the quantum dots have been incorporated into the LED encapsulant, oxygen can still migrate through the encapsulant to the surfaces of the quantum dots, which can lead to photo-oxidation and, as a result, a drop in quantum yield (QY).
The widespread exploitation of quantum dot nanoparticles has been restricted by the nanoparticle's physical/chemical instability and incompatibility with many of the materials involved in such exploitation, such as solvents, inks, polymers, glasses, metals, electronic materials, electronic devices, bio-molecules and cells. Consequently, significant challenges to the development of quantum dot-based materials and methods for fabricating quantum dot-based devices, such as light-emitting devices, on an economically viable scale and which would provide sufficiently high levels of performance to satisfy consumer demand.